Hybrid vehicles and electric vehicles recently use batteries of large capacity and high output power for driving motors. Each battery is formed of a number of battery cells connected in series. Each battery cell is a lithium-ion secondary battery cell, a nickel-hydrogen storage cell or the like. A battery monitor system monitors a battery voltage of the battery cells so that the battery cells do not abnormally over-charge or over-discharge. One exemplary battery monitor system is disclosed in patent document, JP 2008-309747A.
This battery monitor system is provided with plural battery groups, plural monitor integrated circuits (ICs) as monitor modules and a monitor microcomputer as a monitor controller. Each battery group is formed of plural battery cells connected in series. Each monitor IC monitors a battery voltage of the battery group. The monitor microcomputer communicates with plural monitor ICs. According to this system, each monitor IC monitors the battery voltage of corresponding one of plural battery groups and the monitor IC transmits monitor data acquired by monitoring the battery to the monitor microcomputer. In a case that a failure arises in the battery group, the monitor microcomputer analyzes data such as cell voltages transmitted from the monitor IC and, when necessary, stores and hold the data in a non-volatile memory such as an EEPROM connected to the monitor microcomputer.
According to the above-described conventional battery monitor system, the monitor microcomputer is disabled to receive data from the monitor microcomputer, when abnormality such as disconnection arises in any of communication lines connecting the monitor ICs and the monitor microcomputers. Under such a communication abnormality state, the monitor microcomputer cannot store, to the non-volatile memory to be held therein, the data such as cell voltages (data detected by the monitor ICs) detected at the time of failure in the battery group.